As you undoubtedly know pumps are designed to develop head so the pump can be sized to deal with any losses you design on the discharge. The suction however is another story. The suction of a pump generally has less than one atmosphere of pressure available to inject the fluid into the pump properly and you have virtually no control of that. If you get the design of the suction wrong the pump will generally either stall or cavitate. It you get the design of the discharge wrong generally all that happens is that you lose or gain a bit of flow. The suction of the pump is by far the most delicate part of the installation and should be given at least half of your attention to ensure that you get it right.

For accurate control of simple systems it is necessary for the control valve to have a fairly high head loss across it compared to the entire system pressure loss. The ratio of valve loss to total system (including valve) loss is called authority, and for good tight control it should be around 0.5 (1:2) or better. A system with all the head loss across the valve will have an authority of 1. An authority of 0.5 means 50% of the head is lost across the valve, and the remaining 50% is lost across the rest of the system. So for control we want the valve to have a high head loss which means its trim will be small, and the rest of the system to have a low head loss which means big pipes. So thats the reason why you see smaller that line-size control valves, apart from cost of course.

The big problems start when there is very little available head loss for the valve, and we cant have a high authority. This can mean we need a line size control valve but will need to introduce special valve trims, additional feedback control systems and/or characterized control algorithms in order to achieve acceptable controllability . About 50% of the control valves I see are oversized. You can tell an oversized valve because it runs almost closed during normal operation. The prices paid for oversize are poor control, high wear and unnecessarily high first cost.

It's a big question and cant be fully answered in a single posting. Fortunately you will find plenty of good information on the internet, and also in vendors brochures and manuals.

Codes and Standards that have an impact upon piping design. The ASME series of B31 Codes for Pressure Piping have a lengthy history:

The ASME B31 Code for Pressure Piping:

The original ASME B31 Code for Pressure Piping was tentatively introduced in 1935 as a single all inclusive document for piping design. Beginning in 1955, various sections were "split-off" to address the designs of specific piping systems. The group that is now ANSI has in the past been know as ASA and USASI. For a number of years the terminology was ASA/ASME B31 and ANSI/ASME B31 and now just ASME/B31.

With staturated steam, the impulse lines and the wet end of the pressure transducer are usually arranged to run full of condensate. The aim is to have both HP and LP impulse lines full of liquid to an equal height above the transducer so the transducer requires no zero offset and there is no zero drift due to condensation during operation. The impulse lines are often unlagged and sometimes a condensate pot is fitted to each line to provide a reservoir of condensate and help provide stability and a degree of adjustability to the condensate level. Top tappings can be used with condensate pots on saturated steam provided the impulse tube from tapping to pot can drain back to the steam line. The aim is to keep the lines (from pot to transducer) equally full of condensate while running and when shut down for short periods. It would be theoretically possible to arrange a top tapping setup for saturated steam, but any condensation which forms and remains in the impulse tubes or the transducer body would affect accuracy.

With superheated steam any condensate will tend to be evaporated when in contact with saturated steam so a known level of condensate in the impulse tubes cannot be maintained easily. The plan normally adopted for superheated steam is to keep everything hot to minimise condensation, and to arrange for any condensate formed to drain back into the process line. Top tapping with transducer above process line and insulated impulse tubes is an easy way to arrange this.

There's always been a lot of debate about this subject, and often the arrangements seen are a house standard as opposed to a custom arrangement.

Hanger spacing charts for horizontal "runs" of pipe are based upon limiting the bending stress in the horizontal piping (and they DO NOT address concentrated loads like large valves). Supporting vertical piping is quite a different thing.

Before 3-D computer-aided design (CAD) became popularly used to design process plant projects, two-dimensional drawings were created to describe what was to be built. Design studies were conducted by senior piping designers who transformed process and instrumentation diagrams into the lay-out of major process equipment, piping, pipe racks, etc. These studies were then handed off to a senior or intermediate draftsperson who would create plans, elevations and section views, further detailing the design.